A reverse stranding apparatus for the reverse stranding of conductors, such as filaments, conductor elements, bundles of conductors, optical fibres and similar for the manufacturing of cables or the like, comprising a stationary divider means disposed at the upstream end for the conductors to be stranded, a twisting means rotatable in different directions and disposed at the downstream end for the conductors to be stranded, and a medially disposed central element rotatable recurrently about its longitudinal axis in opposite directions and peripheral tubes being twistable recurrently in opposite directions and peripherally surrounding the central element, the central element and the peripheral tubes being disposed between the divider means and the twisting means and being pressed against each other at least during the twisting step of the conductors and the conductors to be stranded being adapted to pass through at least the peripheral tubes.
A wide variety of apparatus as presented above are known for instance in the cable industry. The apparatus disclosed in U.S. Pat. No. 4 974 408 may be mentioned as an example of prior art apparatus in this field. The apparatus disclosed in said U.S. Patent operates completely faultlessly in most circumstances. However, in some circumstances the operation is not the best possible. For example, it has been found in practice that with high speeds of rotation of a tube packet, centrifugal force will produce outward radial displacement of the peripheral tubes on the unbound portions thereof. In the above known solution, the peripheral tubes are bound by means of spaced elastic rings, the compression force of which acts only on a specific longitudinal area of the tubes. The axial spanning force on the tube packet and the relative distance of the elastic rings together define the maximum speed of rotation. Exceeding said speed will have the result that the outer peripheral tubes bulging outwards under the influence of centrifugal force displace the elastic rings in the axial direction, and thus with this mechanism the disruption will be self-augmenting. Also the central tube has a tendency to come out from between the peripheral tubes, if it has been stretched more than the outer peripheral tubes. Another problem is presented by creep, which is due to the properties of the plastics used as the tube material. This creep is very intense at the initial stage of putting into use the tube packet, and consequent thereon the tube packet must be shortened several times. Later said creep will diminish, but it will not cease. On account of the creep, the maximum length of the tube packet is limited, which again limits the maximum relative distance of the reversing points. A third problem worth mentioning is that the desired axial spanning force cannot be used owing to the material employed. This fact is reflected in sagging of the tube packet, which in turn will produce undesired whirling. It is known from experience that a large sag of the tube packet will increase the friction coefficient between the tube and the conductor. The plastics that are most suitable for use as tube materials typically have high friction coefficients and poor mechanical strength values. In the prior solutions, specifically the friction coefficient between the tube and the conductor is the most significant factor affecting the number of upturns in the same direction which is obtained. The intent is to achieve a maximum number of upturns in the same direction, as by this means the reversing points can be disposed far from one another, and this again improves the electrical characteristics of a telecommunications cable.
It is the object of the present invention to provide a reverse stranding apparatus wherewith the drawbacks of the prior art can be eliminated. This has been achieved with the reverse stranding apparatus of the invention, which is characterized in that the peripheral tubes are connected substantially for their entire length to the central element by means of a substantially radial part.
The advantage of the invention lies above all in that the radial connection of the tubes to the central element allows remarkably high speeds of rotation for the rotating head/tube packet. Very high grouping and stranding speeds are possible even with small upturn values. Generally speaking, it can be said that due to the invention, the tube packet will no longer limit the speed of rotation, but the limiting factor is constituted by the motor. The construction of the tube packet in accordance with the invention is safer and more durable than heretofore. At no time will the central element be able to push out from between the peripheral tubes. In the construction according to the invention, the tube packet will retain its initial longitudinal dimension for its entire service life. The axial spanning force on the tube packet can be increased many fold, thus permitting effective prevention of whirling. A smaller friction than heretofore is achieved between the tube and the conductor, since it is known from experience that the smaller the sag, the lower the friction. Thus, the length of the tube packet will not constitute a restricting factor. A further advantage is that by means of the invention, different and continuously varying run profiles of speed of rotation for the rotating head can be employed without any restrictions. The rotating head and the tube packet typically display very high speeds of rotation, continuously changing direction. A small friction between the tube and the conductor will add to the windings of the tube packet in the same direction. A better quality than heretofore is achieved for sophisticated cables. The grouping or stranding procedures do not induce any forces acting adversely on the conductor or any deformation that would stretch the conductor, for instance.